In recent years, the use of advanced composite structures has experienced tremendous growth in the aerospace, automotive, and many other commercial industries. While composite materials offer significant improvements in performance, they require strict quality control procedures in both the manufacturing processes and after the materials are in service in finished products. Specifically, non-destructive evaluation (NDE) methods must be used to assess the structural integrity of composite materials. This assessment detects inclusions, delaminations and porosities. Conventional NDE methods are slow, labor-intensive, and costly. As a result, testing procedures adversely increase the manufacturing costs associated with composite structures.
Various methods and apparatuses have been proposed to assess the structural integrity of composite structures. One solution uses an external source to generate ultrasonic surface displacements in a work piece which are then measured and analyzed. Often, the external source used to generate the ultrasonic displacements is a pulsed laser beam directed at the work piece. Laser light from a separate detection laser is scattered by the ultrasonic surface displacements created at the work piece. Collection optics then collect the scattered laser energy. The collection optics are coupled to an interferometer or other device, and data about the structural integrity of the composite structure can be obtained through analysis of the scattered laser energy.
Laser ultrasound has been shown to be very effective for the inspection of parts during the manufacturing process. In particular, laser ultrasonic testing systems incorporating a two-wave mixing photorefractive interferometer (“TWM”) exhibit advantages over other optical devices for optical demodulation of ultrasonic signals, such as a Fabry-Perot (“FP”) interferometer. The TWM interferometer is more compact and less sensitive to vibrations than the FP interferometer, making the TWM interferometer a better choice for mobile and/or in-field laser-ultrasonic systems.
However, one difficulty with the TWM interferometer is that it requires a reference (or pump) beam in addition to a probe (detection) beam to work. The pump beam must be generated by the same laser source as the probe beam. Moreover, the pump beam typically has peak powers between 10's and 100's of watts for scanning applications. These peak powers make the transmission of the pump beam through an optical fiber difficult over large distances due to effects like stimulated Brillouin scattering. This difficulty of transmitting the pump beam over long fiber distances can be worked around by positioning the TWM interferometer close to the laser source (detection laser). However, such an effective proximity cannot be easily obtained for scanning systems where the space around the detection laser is limited. The injection of a high peak power pump beam into an optical fiber is also a concern because the optical fiber can be damaged by the high powered beam if a misalignment occurs.
Another problem with having a high power pump beam is that power must be diverted from the probe beam to supply the pump beam. Therefore, the more power is diverted to the pump beam, the less power goes to the probe beam. The signal-to-noise ratio of the detected ultrasonic waves is dependent on the amount of light (power) of the probe beam. Therefore, the detection laser power diverted into the pump beam decreases the quality of the ultrasonic signals.
Further, in existing TWM interferometer laser ultrasonic detection systems, the level of pump beam power cannot be controlled independently of the probe beam power and the power of the pump beam influences the time-response of the photorefractive crystal and also contributes to background noise on the detector.